The trough solar collector is a well-known collector technology used for Concentrating Solar Power (CSP) plants. As shown in FIG. 1, such a plant typically employs a large array of sun-tracking, focusing reflectors that concentrate incoming solar radiation onto a tubular conduit that contains a working fluid. The focused radiation heats the working fluid, for example an oil or other fluid. The heated working fluid is piped to a central location where its accumulated thermal energy may be utilized in a conventional heat engine, for example to generate steam that drives turbines to produce electric power. In other applications, the heated working fluid may be used directly, for example where the working fluid is heated water for domestic or commercial use. After its thermal energy has been utilized, the working fluid may be recirculated through the collector array to be heated again.
The collector arrays may be quite large, covering several square kilometers and including thousands of collector modules, such as the module 101 shown in the simplified diagram of FIG. 1. Several modules are shown in FIG. 1, each of which has a similar construction. The field or array of collectors may be divided into parallel circuits, so that the working fluid need not be circulated through the entire collector field before it is piped to the central location, but instead may be passed through a single row of a few dozen modules during each heating cycle, for example. Many arrangements of circuits are possible. Each module typically includes a parabolic reflector 102 backed by a frame or truss system 103 on the back side of the reflector (away from the sun). The frame adds rigidity to the module. The modules are typically supported on pylons 104 that are located between the modules.
The collector modules are typically grouped into rotatable solar collector assemblies (SCAs) of several adjacent modules each, connected in a row. That is, an SCA typically includes several collector modules supported by pylons in a linear arrangement, such that the collector modules in each SCA can rotate about a longitudinal axis. For optimum collection efficiency, all the modules in an SCA preferably rotate in unison to track the sun during the day. Each SCA may be moved by a drive mechanism (not shown) near the center of the SCA, at an end of the SCA, or at another location within the SCA. The collector modules in an SCA may be coupled to each other using a conventional torque transfer assembly that includes a central torsion element (shaft) to couple adjacent modules. Alternatively, adjacent modules may be coupled near their edges or rims, so that torque is transmitted between the modules primarily by a force couple acting at the rim and axis of rotation, rather than by torsion of a central shaft. Preferably, the coupling between modules accommodates thermal expansion and contraction of the SCA. More description of systems and methods for “edge drive” torque transfer may be found in co-pending U.S. patent application Ser. No. 12/416,536 filed Apr. 1, 2009 and titled “Torque Transfer Between Trough Collector Modules”, the entire disclosure of which is hereby incorporated herein by reference for all purposes.
The SCA modules transfer torque from at least two different sources. First, a drive mechanism located near the center of the SCA applies torque directly to those modules adjacent to the drive mechanism. For the rest of the modules in the SCA, torque is coupled from one module to the next so that the entire group of modules in the SCA rotates in unison. Second, the module arrays are also subject to wind loading, which may exert very large forces and torques on the array. Wind loading on each module is transmitted to the adjacent module. The resulting torque may be smallest at the end modules of an SCA, but may accumulate through the modules in the SCA row until the drive mechanism must resist the accumulated torsional wind loading of many modules. The total applied torque may be as large as hundreds of thousands of Newton-meters. In order to maintain proper aiming of the array toward the sun, the drive mechanism must be able to resist and overcome the torque resulting from wind loading, and the SCA must be stiff enough that no modules deflect enough from optimum aiming that their energy collection performance is degraded significantly. While the torques are greatest near the drive mechanism, and the modules adjacent the drive mechanism must resist the largest torques, the deflection may accumulate outward from the drive mechanism, and may be greatest at the end of the SCA furthest from the drive mechanism.
In order to achieve enough stiffness, the frame or truss system 103 should be designed to withstand the expected torques with acceptably small deflection. Also, the coupling of two or more optically-precise devices, such as the modules of an SCA, requires that the assembly be fabricated with a relatively high degree of precision for proper energy collection. In addition, it is desirable that each module be light in weight, easy to assemble, and low in cost. In large part, these competing design goals—stiffness, accuracy, light weight, ease of assembly, and low cost—are dependent on the design of the frame or truss portion of the collector modules. There is accordingly a need for improved frame designs for use in solar collector modules.
Because the efficiency-over-time of CSP systems is strongly dependent on how much time the reflectors can be exposed to sunlight, CSP systems are usually located in high sunlight areas, which are also usually high temperature environments. These environments are often low-cost desert locations which combine the high amounts of sunlight with large amounts of space to locate many mirrors from which to collect solar energy.
Unfortunately, while deserts provide excellent environments for the collection of solar energy, these same environments are usually detrimental to the physical structures necessary to do so. To collect a significant amount of solar energy for the heat engines, large surface areas of mirrors are necessary. The most economical method of providing and placing the requisite large surface areas of mirrors is to use fewer larger mirror panels rather than many smaller mirror panels. However, as the size of the mirror panels increases, the physical stresses on these mirror panels also increases, due to both the weight of the larger mirror panels themselves, as well as the structures necessary to support them. The high temperature environments also magnify the physical stresses, and over time can distort the precise concave shapes of the mirrors that are necessary to achieve maximum solar reflection to the collection means.
An improved frame structure would provide good torsional strength and stiffness, as well as strength an stiffness in bending, while utilizing material efficiently.